1. Field of the Invention
The present invention relates to a powder of spherical glass particles and a method for manufacturing the powder.
2. Description of the Related Art
Conventionally, the most popular method for manufacturing a glass powder is a melting-quenching-crushing method. That is, the main raw materials (silicon oxide, alumina, alkali metal compounds, etc.) constituting a glass, and auxiliary raw materials affecting the melting point and physical properties (crystallization temperature, coefficient of thermal expansion, dielectric constant, etc.) of the glass, are both fed into a vessel such as a platinum crucible, are heated and melted in a melting furnace at a temperature 200 to 300° C. higher than the melting point of the glass (a temperature at which the viscosity of the glass is not more than 103 to 104 dPa/sec) so as to become fully homogeneous and clarified, and then are poured into water to be quenched for vitrification. Then, a glass powder is formed by crushing the glass.
Since the glass powder according to the conventional method is formed by mechanical pulverizing, the obtained powder particles have indeterminate shapes. When such a glass powder is used, for example, for manufacturing a laminate comprising an electroconductive layer and a non-conductive layer (particularly a laminate having a thin electroconductive layer), there will occur some problems. For example, sharp edges of the indeterminate glass particles would break through the electroconductive layer. Also, there are other problems regarding a molded article using a powder of indeterminate glass particles in that the glass particles tend to cause voids in the article and the density of the article tends to drop.
In comparison, spray-pyrolsis method for manufacturing a powder of spherical glass particles which avoids such problems is disclosed in Japanese Unexamined Patent Application Publication No. 8-91874. It teaches that spray-pyrolsis method in which a thermal decomposition reaction of the sprayed droplets is caused by radiant heat, provides hollow glass particles, while spray-pyrolsis method in which droplets are sprayed into a flame environment, provides a powder of solid spherical glass particles.
When the sprayed droplets are thermally decomposed by the radiant heat, the temperatures of the droplets themselves are raised. However, the carrier gas is hard to heat by the radiant heat, and therefore the gas temperature is not raised greatly. Accordingly, the gas flow which is taken out from the spray-thermal decomposition furnace and which contains glass melts (precursors) can be easily cooled. This method has an advantage that a wide compositional range of raw materials is applicable for vitrification although the glass produced is hollow.
When the sprayed droplets are subjected to a thermal decomposition reaction in a flame environment, the carrier gas itself is heated to a very high temperature by the burning. Thus, it is necessary to dilute the high-temperature carrier gas with quite a large amount of a low-temperature gas in order to rapidly cool the gas, and therefore it is difficult to rapidly cool the glass melts contained in the carrier gas. Accordingly, there is a problem of a limited compositional range of raw materials allowed for vitrification.